1. Field of the Invention
The present invention relates to improvements in a position detecting device and a position pointing device therefor, which employ electromagnetic waves. This application is copending with patent application Ser. Nos. 08/318,171 and 08/359,463.
2. Description of the Related Art
Prior to the instant application, the applicant has proposed, in Japanese Patent Application No. 61-213970 (Japanese Patent Laid-Open No. 63-70326), a position detecting device in which a tablet exchanges electric wave signals with a position pointing device so as to determine the coordinate value of a position pointed by the position pointing device.
Briefly, the position detecting device disclosed in Japanese Patent Laid-Open No. 63-70326 has a plurality of loop coils constituting the tablet and is adapted to transmit an electric wave of a predetermined frequency. The electric wave is received by a resonance circuit provided in the position pointing device so that resonance takes place in the resonance circuit to emit electric wave. This electric wave is received by the loop coil so that a voltage is inducted in the loop coil. This operation is conducted successively on the plurality of loop coils constituting the tablet, so that the coordinate value of the position pointed by the position pointing device is determined based on the levels of the voltages induced in the loop coils.
The position detecting device of the type described faces a demand for the ability to enter various kinds of information in addition to the coordinate value of a pointed position, such as information which indicates that the position pointing device, e.g., a pen or the like, has been set to a position for pointing a definite position (this state will be referred to as "pen-down state," hereinafter), information concerning the type of the position pointing device, e.g., a pen, a cursor or the like, as well as a demand for inputting, together with the coordinate values, information for continuously varying values of parameters other than the coordinate information, such as, for example, thickness or width of a line and hue or density (brightness) of the position or area appointed by the pointing device.
To cope with such demands, the position pointing device disclosed in Japanese Patent Laid-Open No. 63-70326 employs a manual switch which selectively connects, to the resonance circuit which is constituted by a coil and a capacitor, an additional capacitor so as to slightly vary the resonance frequency, the slight change in the resonance frequency being detected as a variation in the phase angle so as to be used as information indicative of the aforesaid various types of information (hereinafter referred to as "pointing device information").
The device according to the aforesaid Japanese Patent Laid-Open No. 63-70326 requires, however, that the range, over which the resonance frequency of the resonance circuit varies as the switch is operated, precisely coincide with a predetermined range, which is centered at the frequency of the electric wave transmitted from the loop coil. A laborious adjusting work is required to attain such matching of the frequency ranges. Further, the detectable range of phase angle is limited to be about -60.degree. to about +60.degree.. Furthermore, a certain tolerance for the phase angle to be detected has to be set in order to accommodate any change in the inductance of the coil and in the capacitance of the capacitor of the resonance circuit caused by a change in the ambient temperature, etc. These problems led to a limited number of types of pointing device information, which can be entered, and especially when the resonance frequency is designed to continuously change according to pen pressure, no pointing device information other than pen pressure information can be entered.
Under this circumstance, the same applicant proposed a position detecting device and a position pointing device therefor in Japanese Patent Application No. 1-327276 (Japanese Patent Laid-Open No. 3-189716) and Japanese Patent Application No. 1-327277 (Japanese Patent Laid-Open No. 3-189717), wherein the position pointing device is provided with a code generator, which issues a plurality of particular bits of binary code according to the operation of a switch, an additional capacitor is connected and disconnected to and from the coil and capacitor composing a resonance circuit via an electronic switch in accordance with a plurality of bits of binary code issued from the code generator in order to cause the resonance frequency to change slightly according to the plurality of bits of binary code, and the slight change in the resonance frequency caused by the binary code is detected as a change in phase angle to reproduce the binary code, thereby making it possible to enter a number of pieces of pointing device information, which corresponds to the number of the bits of binary code.
In the position detecting device described above, however, a change in the resonance frequency of the resonance circuit causes a change in the level of a received signal (induced voltage); therefore, accurate detection of a coordinate value cannot be performed if the resonance frequency changes according to the aforesaid binary code during coordinate detection.
To cope with the problem, the device according to Japanese Patent Laid-Open Nos. 3-189716 and 3-189717 is provided with a preset period for coordinate detection and a preset period for identifying a code, the two periods alternating. The tablet transmits an electric wave, which includes timing information for distinguishing these periods, to the position pointing device. The resonance frequency of the resonance circuit is changed in accordance with the plurality of bits of binary code at a certain timing specified by the timing information, thereby supplying (transferring) the pointing device information represented by the plurality of bits of binary code from the position pointing device to the tablet.
The frequency at which the pointing device information is required varies, depending on the type of the aforesaid pointing device information or a host system (or a program involved), which processes the pointing device information together with the coordinate value of a pointed position. In the case of the devices according to Japanese Patent Laid-Open Nos. 3-189716 and 3-189717, the transfer of any pointing device information alternates with the transfer of the coordinate value of a pointed position. This leads to wasteful time spent for information transfer, resulting in a lower sampling rate for coordinate detection.
When transferring, for example, the information on a switch operation state and a pen pressure, which are completely different types of pointing device information, from a single position pointing device to the tablet, it is necessary for the devices according to Japanese Patent Laid-Open Nos. 3-189716 and 3-189717 to switch between the output of the code generator, which generates a binary code corresponding to the operation of the switch, and the output of an analog-digital converter or other device generating a binary code corresponding to the pen pressure in order to apply the outputs to the foregoing electronic switch. This posed a problem of inefficient operation because the operator must manually switch the outputs.